It wasn't until I tried the Shake Shack's 'Shroom Burger that I realized veggie burgers needn't be sensible. Why coax vegetables (and, for most recipes, grains and binders and starches, too) into the shape of a patty? Vegetarian burgers can be entirely nonsensical and untraditional and just as messily delicious as any meat burger on the menu.
The 'Shroom Burger forgoes the constraints of the patty form to present a mushroom in its most dignified state—breaded, fried, full of cheese. It's insanity! When you eat the sandwich, you work your way through the roasted mushroom barrier to hit the cheese geyser at its center.
And you needn't purchase a specialty cheese syringe (oh yes, these exist) to make them. Simply roast the mushrooms, slice them in half through their bellies, then hide a ball of the filling—a mixture of grated cheese, onion, garlic, flour, egg yolk, and cayenne—in between the top and bottom. Coat the reconfigured mushrooms in flour, beaten eggs, and panko, then get them into the hot oil.
For the mushroom caps, preheat the oven to 375° F. Put mushroom caps on a medium baking pan. Rub caps all over with 1/4 cup of canola oil, then season both sides with the salt and pepper. Arrange mushrooms gill side down in a single layer and roast until tender, 30 to 35 minutes. Remove from the oven and set aside to cool. Slice mushrooms in half horizontally (through their bellies, like you're slicing a hamburger bun).
Put the 1 1/2 cups flour in a wide dish, beat 3 eggs in another wide dish, and put the panko in a third wide dish and set aside.
For the filling, mix together the Muenster and cheddar cheeses, onion, garlic, remaining 1/4 teaspoon flour, remaining egg yolk, and cayenne pepper in a medium mixing bowl. Set aside.
Shape the filling into 4 equal disks, 1/2 inch smaller in diameter than the mushroom caps. Place cheese disks on gill side of each mushroom cap, cover with other half of cap, cut side down, and gently press together to secure the filling.
Dredge the stuffed mushroom caps in the flour, then dip in the beaten egg, coating them completely, then dredge in the panko. Set prepared mushrooms on a plate and refrigerate while heating the oil to deep-fry them.
To cook the mushroom burgers, pour the oil into a deep, wide pot to a depth of 3 inches. Heat over medium heat until the oil reaches a temperature of 350° F on a candy thermometer. Deep-fry the prepared mushrooms, one or two at a time, until golden brown and crisp all over, about 3 or 4 minutes. Transfer mushrooms with a slotted spoon to drain on paper towels. Season with salt.
Spoon the ShackSauce onto the top bun. Add a piece of lettuce and two slices of tomato. Transfer the mushroom burgers to the prepared buns. Enjoy!
A (former) student of English, a lover of raisins, a user of comma splices. My spirit animal is an eggplant. I'm probably the person who picked all of the cookie dough out of the cookie dough ice cream. For that, I'm sorry.